In particular for meat, fish, poultry and dry (ready-to-eat) products such as sandwiches, burgers or wraps, transparent ready-meal trays are used which are made of thermoformable polyester, for example a-PET or modified boPET, are produced with use of vacuum and, after filling with food, are closed by a foil heat-sealed onto the perimeter of the ready-meal tray. Provision of the products in such packaging is regarded as clean and hygienic, and is therefore very popular. Vacuum processes, e.g. using Multivac machines, is a cost-effective method of producing hygienic packing for meat, fish or poultry. The finished packing is generally produced here as follows: A roll of the foil for producing the ready-meal tray—also termed lower foil—is clamped into the front region of the machine. By way of one or more vacuum chambers, the foil is thermoformed to a desired depth by use of heat and vacuum. According to the prior art (see, for example, EP 1 697 129 B1), depths up to about 70 mm can be achieved, for example when laminates made of polyester and polyamide are used. After thermoforming, the food product is placed into the ready-meal tray, mostly by hand. By way of a further roll, the lid foil or upper foil is introduced to the ready-meal tray and securely sealed to its perimeter, with use of heat and pressure. This step mostly again takes place under vacuum, which is applied to the entire packing. This is followed by separation into individual packs and other associated separate downstream operations, for example the printing or labelling of the packaging.
The packs are frozen and passed onwards for sale. The consumer defrosts the pack and cooks the same in an oven at temperatures up to 220° C. for a defined time (about 30 min to two hours). To this end, the pack is placed, closed, into the preheated oven (cook-in), where it automatically opens at the seal seam—ideally after from 5 to 10 min (the term used for this being self-venting).
Self-venting is an essential feature in the development of new packs with improved functionality or with modified design. The automatic opening of the seal seam, in essence as a consequence of the increased pressure of steam in the pack, allows the steam to escape from the pack, and the product becomes crisp and brown—as desired by the consumer. Once the cooking time has expired, the upper foil is peeled—while retaining its integrity—from the ready-meal tray while it is still hot, and the contents of the pack are available for consumption.
The pack of the invention, produced from the two foils, exhibits the desired self-venting during cooking. Self-venting is achieved when the sealing properties of the upper foil have been adjusted appropriately for the lower foil.
The market places stringent optical requirements on the two foils of the pack, in particular on haze, clarity and gloss. For good discernibility of the contents of the pack (size of the food), the haze of the pack should be <5% and the clarity of the pack >85%. The pack is moreover intended to be visually attractive. To this end, high gloss values of the pack are desirable.
High puncture resistance is also demanded from the pack, in order that it can be transported without damage.
Packs made of thermoformable lower film and (sealable) upper film are known.
EP 1 697 129 B1 describes a thermoformable foil laminate which comprises a thermoformable foil layer (a), a structural foil layer (b), a heat-sealable layer (c) and optionally a barrier layer (d). The structural foil layer (b) is adjacent to the first surface of the thermoformable foil layer (a), and the heat-sealable layer (c) is on the opposite, second surface directed towards the internal side of the pack. The thermoformable foil layer (a) comprises a polymer composition comprising at least 80% by weight of polyethylene terephthalate. The structural foil layer (b) can comprise a large number of polymers, and preferably comprises a polyamide. The heat-sealable layer (c) can likewise comprise a large number of different polymers, and preferably comprises an amorphous polyester. The laminate is suitable for the application mentioned in the introduction, but requires improvement in thermoformability and puncture resistance, and especially requires improvement in optical properties such as haze, clarity and gloss.
EP 1 945 512 B1 describes a process for the packaging of fish or meat which in essence comprises the steps mentioned in the introduction of the present application: (i) provision of a thermoformable polymeric holder film (=lower foil) and of a polymeric cover film (=upper foil). The holder film consists of a single-layer polyester substrate or polyamide substrate, and the cover film is preferably comprised of polyester polymers. At least one of the two surfaces of the holder film and of the cover film is heat-sealable. (ii) Provision of an elevated exterior portion and of a lowered central portion in the holder film via thermoforming. (iii) Arrangement of a portion of meat or fish on the interior (=first) surface of the holder film. (iv) Arrangement of the cover film over the portion of meat or fish, so that the interior (=first) surface of the cover film faces towards the interior surface of the holder film. (v) Bringing the perimeters of the first surface of the holder film and of the first surface of the cover film into contact and formation of a heat-seal bond therebetween, and (vi) optionally freezing of the packaged meat or fish. The process is suitable for the application mentioned in the introduction; however, the foils described in more detail in EP 1 945 512 B1 require improvement in thermoformability and puncture resistance, and especially require improvement in optical properties (haze, clarity and gloss).
Another example is provided by EP 0 415 383, which describes the use of a polyester foil with a melting point from 210 to 245° C., with planar orientation of from 0.10 to 0.16, with shrinkage smaller than or equal to 10% and with density below 1.385 g/cm3. The foil features good formability and good adhesion to metal and is intended for use by way of example in lamination to metal containers. The process is suitable for the use mentioned in the introduction; however, the foils described in EP 0 415 383 require improvement in optical properties (haze, gloss and clarity).